Started refactoring the way the download works.
Changed the md5_verify() so that it uses "md5sum -c"
Added popd to the md5_verify()
Removed obsolete code from md5_verify()
Added mirrors.slackware.com to the mirrors list

See http://mirrors.slackware.com/
There, I fixed it.

...Well, at least to a certain point.

Now the directory structure inside ${WORK_DIR} should be the same than
that in ftp://ftp.slackware.com/pub/slackware/slackware-${VERSION}/.

There's probably bunch of new bugs introduced now, but at least the
basic functionality should be working again.
Fixed the get_file() descrition regarding the parameters
Commented out most of the temp debug output
Changed the MAIN_MIRROR
Commented out debug output line.
Changes relating to the PGP signature timestamp functionality.

- Fixed the path "${WORK_DIR}/CHECKSUMS.md5.asc" ->
  "${WORK_DIR}/patches/CHECKSUMS.md5.asc"
- Changed the one if to to ifs, so that we can also echo that we're
  comparing the timestamps
Commented out few debug output lines.
Added few TODOs
Improved debugging possibility with PS4
Changed the mirrors
Renamed a function to imply that it's not being used
Changed a few echos

Since we don't (re-try to download the files anymore in this function.
Removed a commented line
Changed one echo line

From "..." to ":", since the output will follow.
Output the MD5SUM option value from installpkg in print_configuration()